Stamping presses are power-driven machines useful for stamping or otherwise shaping metal and other material by heavy blows. A stamping press comprises upper and lower dies disposed respectively between a slide and a bed. The slide, and therefore the upper die, is driven up and down by a motor. The motor is connected to the slide by way of a flywheel and a crankshaft. The motor continuously drives the flywheel and, when a clutch/brake mechanism is engaged, the flywheel turns the crankshaft which moves the slide up and down. When the clutch is disengaged, the brake engages to stop movement of the upper die. In use, sheet metal is placed between the upper and lower dies and the upper die descends rapidly to stamp the sheet metal.
During operation of the stamping press, it is important that the stopping of the upper die be precisely (or repeatably) measured and controlled. Two separate problems have been identified in this regard.
The first problem has been to provide a convenient way to precisely measure the brake time of the stamping press. The brake time is the amount of time that it takes the upper die to stop moving once a stop command has been issued to the brake. Safety standards (specifically, OSHA 1910.217) require that the brake time of a stamping press with an air activated clutch/brake be periodically measured in order to detect brake wear before it becomes dangerous to operate the stamping press.
Two approaches are currently used for measuring brake time. The first (more common) approach involves attaching a retractable string to the upper die using a magnet. The retractable string is on a spring loaded spool located inside a black box, and retracts back into the black box as the upper die travels downwardly. The black box is also connected to the stamping press control system. The black box calculates the brake time based on the amount of time that the flywheel continues to rotate (as indicated by the movement of the string) after a stop command is issued (as indicated by an input from the control system).
The second approach involves connecting a black box to the control system and to a resolver or an encoder which is attached to the flywheel. In this case, the black box calculates the brake time based on the amount of time that the slide, or die, continues to move (as indicated by the encoder/resolver) after a stop command is issued (as indicated by the input from the control system).
Both of these approaches suffer from several disadvantages. First, these approaches require extra equipment (the black boxes) which do nothing other than measure the brake time of the stamping press. The use of extraneous equipment in this manner is undesirable. Further, these approaches are primarily adapted for making only occasional measurements of brake time. These approaches are not intended to be used for measuring brake time every time the stamping press stops, which would enhance the ability to perform predictive maintenance. Moreover, the precision of these approaches is fixed; the black box devices currently in use do not provide the user with a way of adjusting the precision of the brake time measurement. Finally, the string approach is especially disadvantageous because the string bounces when the upper die stops, thereby making it appear as though the upper die is still moving when in fact it has stopped.
A second problem has been to provide a repeatable way of positioning the upper die of a stamping press, and especially a way of positioning the upper die at the top position in a high speed press. The top position refers to the uppermost position of the die, also referred to as the 0.degree. position. (The upper dies starts movement at 0.degree., then travels downwardly past the downward half-way position at 90.degree. and to the bottom position at 180.degree., and then travels upwardly past the upward half-way position at 270.degree. and returns to the top position at 0.degree..)
When performing a top stop, the clutch is disengaged and the brake is engaged causing the upper die to come to an eventual stop. Of course, however, the upper die does not come to an immediate stop. Thus, in order to cause the upper die to come to a stop at the top position, the stop command must be given in advance of the time at which the upper die is at the top position. More specifically, the stop command must be timed such that, once braking occurs, the upper die comes to a stop at the top position.
Typical (non-high speed) presses issue the stop command in the range 200.degree.-355.degree., depending on the speed of the press. Thus, when the upper die reaches a predetermined top stop angle (e.g., 340.degree., assuming it takes the upper die 20.degree. to stop), a stop command is issued, and the upper die comes to a stop at approximately the top position.
To determine whether the upper die has reached the top stop angle, a takeover cam mechanism is utilized. The takeover cam is a limit switch which turns on when the upper die reaches 180.degree. and turns off when the upper die reaches the top stop angle. Thus, the top stop sequence for a typical press may be as follows: First, the upper die reaches 180.degree. and the takeover cam turns on. Then, the operator removes his hand from the actuation button and a top stop input is generated. However, since the takeover cam is on, the control system recognizes that the upper die has already reached 180.degree. and thus continues to permit the upper die to move upward (since there is no threat posed to the operator once the upper die reaches 180.degree.). When the upper die reaches the top stop angle, the takeover cam turns off. The control system detects that the takeover cam has turned off, and that a top stop input has been generated, and in response the control system issues a stop command.
In a high speed press, it is necessary to issue the stop command much earlier (i.e., in the 0.degree.-180.degree. range) because it takes the upper die a longer period of time to stop. Thus, since the stop command must be issued before the upper die reaches 180.degree., the above-described sequence of events is not possible and it is not possible to use the above-described takeover cam mechanism in conjunction with a high speed press. Accordingly, an alternative top stop system for high speed presses is needed.
Efforts to provide an acceptable alternative top stop system for use with a high speed press have met with considerable difficulty. Specifically, it has been difficult to provide a way to issue the stop command when the upper die is approximately at the top stop angle. The difficulty arises because the instantaneous angle of the upper die is usually not known. The angle of the upper die is not monitored continuously but rather only at periodic intervals, and thus the control system is unlikely to know the precise instant at which the upper die is at the top stop angle for a given stroke. (It may be noted that, depending on the length of the intervals, the angle of the upper die may change as much as 30.degree., and perhaps more, between intervals.) This introduces a significant and undesirable amount of variability as to the stopping point of the upper die in a high speed press. Thus, whereas control systems for typical presses are able to stop the upper die at the top position with a .+-.2.degree. variance, conventional control systems for high speed presses are only able to stop the upper die with .+-.15.degree. variance.
Thus, what is needed is a way to measure the brake time of a stamping press which does not require any additional equipment, which can be conveniently performed each time the upper die of the stamping press stops, which can have varying degrees of precision, and/or which does not suffer from the inaccuracies associated with string bounce. What is also needed is a precise way of positioning the upper die of a stamping press, and especially a way of positioning the upper die at the top position in a high speed press.